This invention relates to an improvement of a ceramic precombustion chamber construction for an internal combustion engine, and more particularly to a ceramic precombustion chamber construction fixed by a metal ring fitted thereon.
It has been known in the art to form a precombustion chamber for an internal combustion engine such as a diesel engine by a ceramic material to improve its heat-resistance and thermal efficiency. Such a ceramic precombustion chamber construction has been disclosed, for example, in U.S. Pat. No. 4,616,611 (European Patent Application Publication No. 186,943).
FIG. 1 illustrates the ceramic precombustion chamber construction for an internal combustion engine disclosed in the U.S. Pat. No. 4,616,611. The ceramic precombustion chamber construction comprises a metal ring 2 press-fitted in a cylinder head 1, and ceramic bodies 3 and 4 fixed in the metal ring 2 through thermal shrinkage fitting. The ceramic bodies 3 and 4 are hollow bodies having circular cross-sections to form the precombustion chamber. The upper ceramic body 3 is formed with a through-aperture 6 communicating with a glow-plug receiving portion 5 in a cylinder head 1 and a fuel injection aperture 8 communicating with a fuel injection nozzle receiving portion 7 in the cylinder head 1. On the other hand, the lower ceramic body 4 is formed with an injection aperture 10 for injecting a fuel gas into a cylinder chamber 9. The ceramic bodies 3 and 4 are prevented from rotating by using keys 11 and 12 located in notches formed in outer surfaces of the precombustion chambers 3 and 4 and in an inner surface of the metal ring 2 as shown in FIG. 1.
However, with such a ceramic precombustion chamber construction, large tensile stresses occur in the ceramic chambers near the peripheries of the keys thus often causing cracks in the ceramic chambers.
The reason why the cracks occur will be considered in more detail referring to FIGS. 2-4. A hatched portion A in FIG. 3 is subjected to the thermal shrinkage pressure between the ceramic body 21 and the metal ring 22, while a portion B in FIG. 3 is not subjected to the thermal shrinkage pressure because it is notched for receiving the key 23. On the other hand, a stress concentration occurs in an edge portion C between the portions A and B to often cause cracks in the ceramic body.
Moreover, as shown in FIGS. 6 and 7 the worked accuracy of the notches for receiving the keys is not always sufficient and a tensile stress concentration occurs at an edge 26 of the ceramic body 21 in contact with the metal ring 22 to cause cracks in the ceramic body.
The inventors of the present application have investigated the cause of cracks in ceramic precombustion chambers of the prior art and have discovered the following problems when using conventionally shaped keys as shown in FIG. 6.
It is desired to make key ways 24 and 25 formed in the metal ring 22 and the ceramic body 21 coincident with an outer circumferential shape of a key 23. However, as working methods for a ceramic material and a metal are different due to the difference in material, it is very difficult to obtain a sufficient accuracy in key ways when the metal ring and the ceramic body are assembled. A slight error in worked portions causes a positional difference in key ways as shown in FIG. 6 on an exaggerated scale. When edges 26 of the key way 24 of the metal ring 22 are positioned on a surface of the ceramic body 21, large tensile stresses occur on surfaces of the ceramic body 21 in the proximity of the edges 26 to cause cracks in the ceramic body 21.